Previous aerial cameras employ exposure meters and control units operating on an integral measuring principle, that is, a single photodetector is provided to detect the brightness of an entire terrain to be photographed.
In some applications the spectral sensitivity of the photodetector is adapted to the spectral sensitivity of a film material by using suitable means such as filters.
When color filters are used in the course of photographing a terrain, correction values are manually fed-in which are based on experience or taken from the operator's handbook delivered by the producer of a respective device. The integral measuring method is disadvantageous for the following reasons:
The measuring results are influenced by the random distribution of bright and dark areas in a terrain in the detection range of the photodetector; in particular, an overevaluation of the bright areas, which results from a logarithmic conversion of the brightness values into densities of the negative film, can involve an underexposure of dark areas at a particular distribution.
Furthermore, no information about the entire brightness of an object to be photographed can be obtained from the measuring values, due to the considerably wide detection angle of the detector. Therefore the operator's experience determines the selection of the optimum parameters for the film development in order to completely convert the brightness scale of an object in a terrain into respective discernable densities, that is, to eliminate over- and underexposures. Furthermore, when steep color wedges are used as filters to eliminate any atmospheric mist and, thus, to increase the contrast, the latter is not measurable by the integral measuring method.
In order to eliminate the above disadvantages, W. Komarek in "Modern Technology of Production of Aerial Photograph and Some New Devices Used in Aerial Photography" Int. Arch. Phm XVII Lausanne, 1968, proposed the use of exposure meters which differentially scan a swept terrain and which indicate both the minimum and the maximum brightness and, hence, deliver information which enables the operator to select optimum film exposure and development parameters. This solution has the disadvantage again that the experience of the operator plays the decisive part in determining the development parameters.